This invention generally relates to casks for transporting radioactive materials, and is specifically concerned with an improved double-lidded closure capable of closing and sealing such a cask without the application of rubbing or scraping forces on the gasket seal between the cask and the closure.
Casks for transporting radioactive materials such as the waste products produced by nuclear power facilities are known in the prior art. The purpose of such casks is to ship radioactive wastes in as safe a manner as possible. Such casks may be used, for example, to ship high-level vitrified waste cannisters to a permanent waste isolation site or fuel rods to a reprocessing facility. At the present time, relatively few of such transportation casks have been manufactured and used since most of the spent fuel and other wastes generated by nuclear power plants are being stored at the reactor facilities themselves. However, the availability of such on-site storage space is steadily diminishing as an increasing amount of fuel assemblies and other wastes are loaded into the spent-fuel pools of these facilities. Additionally, the U.S. Department of Energy (D.O.E.) has been obligated, by way of the National Waste Policy Act of 1983, to move the spent-fuel assemblies from the on-site storage facilities of all nuclear power plants to a federally operated nuclear waste disposal facility starting in 1998.
While the transportation casks of the prior art are generally capable of safely transporting wastes such as spent fuel to a final destination, there is a need for improvement, particularly with respect to the closures used to close and seal such casks. However, before these areas of potential improvement can be fully appreciated, some understanding of the structure and operation of prior art closures is necessary.
The primary closure for a typical prior-art transportation cask is generally formed from a circular lid which is attached over the open end of the cask by twenty-four bolts. The threaded ends of these bolts are received in twenty-four bores uniformly provided around the circumference of the lid near the outer edge thereof. Additionally, a bolt ring welded around the open end of the cask includes twenty-four threaded bores which are registerable with the ends of the bolts when they are extended completely through the closure lid. To effect a fluid-tight seal between the lid and the bolt ring, the lid is circumscribed by a gasket or o-ring of resilient material. In operation, the twenty-four closure bolts are inserted into the bolt bores around the lid. The lid is then hoisted over the bolt ring of the casks, and the threaded ends of the bolts extending through the lid bores are carefully aligned with the threaded bores in the ring. The bolts are then screwed into the threaded bores of the bolt ring in order to effect the closure. However, to insure that a uniform engagement force is applied around the ring-like gasket sandwiched between the outer edges of the lid and the bolt ring, the twenty-four bolts must be tightened in accordance with an intricate torquing pattern, wherein bolts oppositely disposed from one another across the cask lid are simultaneously tightened. Implementation of such a torquing pattern requires a significant expenditure of the time of the personnel in charge of closing the casks, which in turn causes them to be exposed to some amount of radioactivity. Moreover, if any of the threaded holes in the bolt ring of the casks should become damaged (as, for example, by an inadvertent over tightening of a particular bolt) the entire cask could become unsealable and hence useless since the bolt ring is essentially a non-replaceable part of the cask, being permanently welded to the cask walls. Still another drawback of bolt-ring closure designs is the fact that they are often not as structurally strong as the cask walls themselves. Thus, they often provide the weakest point in the overall cask structure which is the most likely to break in the event of an accident. A fourth significant drawback to this prior art closure design is the fact that significant wiping and scraping forces are often applied to the gasket as the bolts in the lid are being azimuthly aligned with the threaded bolt holes in the bolt ring. Such scraping or wiping forces can seriously jeopardize the sealing ability of the gasket, and will, at the very least, accelerate its wear-out. Finally, the minimum diameter of the bolt ring is smaller than the diameter of the mouth of the opening it circumscribes. Consequently, a bolt ring often provides an unwanted lip or flange around the opening that interferes with the loading and unloading of the cask.
Clearly, what is needed is a closure for a cask assembly which corrects all of aforementioned drawbacks associated with the prior art. Ideally, such a closure should avoid the application of scraping or wiping forces to the gasket seal in order to prevent the premature wear-out of the gasket. Moreover, such a closure should be rapidly attachable to the open end of the casks so as to minimize the exposure of the personnel implementing the closing of the casks to potentially harmful radiation, and devoid of any unwanted lips or flanges that interfere with the loading and unloading of the cask. Finally, it is desirable that the closure be at least as strong as the cask walls themselves, and capable of maintaining an effective seal around the casks in the event of an accident which applies substantial shock forces to the casks.